In the electrostatic application of paint in the automotive finishing industry, paint may be delivered to a robotically maneuverable atomizer applicator from a plurality of sources, each source providing a different color paint. During application, a high voltage is imposed on the paint, which imparts positive charges on the atomized paint droplets, which are then uniformly attracted to grounded articles being coated, all in known assembly-line fashion. Water based paint is, generally, electrically conductive. Conductivity of the paint composition can create critical safety concerns and hazards in electrostatic operations, wherein the applicator itself must be maintained at a high voltage.
Concomitantly, recently enacted environmental constraints are exerting pressure on the automotive industry to reduce the amount of volatile organic compounds (VOCs) being released into the environment, the majority of which is produced by paint operations. To reduce VOC emissions, painting facilities have gradually been converting from solvent paint carrier systems to water based systems. Water based paint, although much lower in VOC content, creates another set of problems resulting from the electrostatic charges placed on the paint as it leaves the spray atomizer. The charge can travel back down through the electrically conductive paint, acting as a circuit, to ground, thereby presenting a safety concern. The solution is to completely isolate the charged conductive paint being atomized from ground during the painting process.
Several attempts have been undertaken to isolate the high voltage required for electrostatic painting from ground when using conductive, water based paint. One such system to provide galvanic isolation is disclosed in U.S. Pat. No. 4,785,760. This patent describes an electrostatic system for spraying conductive paint wherein a quantity of paint required to paint an object such as a vehicle is stored in a storage tank carried by a multi-axis robot. The spray applicator may be of the rotational, bell cup variety or the pneumatic or hydrostatic spray gun type. The sprayer, carried by the robot, is supplied during painting from a storage tank also mounted on the robot arm and connected to the sprayer. The high voltage generator itself is controllable, that is, its output voltage may be reduced to zero at any time and then re-established virtually instantaneously using conventional controls. See, e.g., the '760 patent at col. 5, line 16 et seq. The high voltage is reduced to zero before a color change cycle is initiated, the storage tank is filled at a local dispensing port, and the voltage is re-established after filling as spraying begins again, resulting in isolation of the high voltage during spraying from the various distribution circuits that are all electrically grounded, because there is no conductive paint conduit connecting them during the spraying operation. ('760, col. 6, lines 57–65).
U.S. Pat. No. 5,310,120 discusses the '760 patent and discloses an alternative storage tank for an electrically conductive liquid coating product. This patent discloses a procedure wherein the coating product, at a high voltage, is carried within a storage tank defined by a substantially cylindrical cavity formed in an insulative material body inside of which is a piston forming a mobile wall separating a coating product chamber from an actuation chamber filled with an electrically insulative actuation fluid. During spraying, because the tank is separated and isolated from the grounded robot carrying it, the electrostatic charge placed on the paint within the storage tank will not track back to ground.
In one further known operation for providing such isolation, an intermediate storage tank is filled with sufficient paint required for one application. Each color change requires that the intermediate tank and the conduits leading to it be cleaned, for example as disclosed in French patent No. 2,572,662. Galvanic isolation is re-established after filling the intermediate tank by draining and drying a sufficient length of conduit upstream of the intermediate storage tank and then commencing painting. This method, however, is said to require a “prohibitive length of time” on each color, and therefore to be “not practical”. See, e.g., U.S. Pat. No. 4,785,760, at col. 3, line 25 et seq.
As a point of reference, the '760 patent suggests providing the necessary galvanic isolation by a method involving robotically picking up one small storage tank 22, spraying its contents, and then “hanging it up” locally and getting another one. See, e.g., '760 at col. 7, line 62 et seq.
While such apparatus and procedures may isolate the charged spray paint, they are generally inefficient. Time is of the essence on the paint finish line, an entire vehicle being painted typically in 2–3 minutes. Travel time in these operations in manipulating storage tanks robotically around a paint room is costly. The more time that is spent in emptying, cleaning and color changing, the more costly is the process. It is therefore beneficial to have a system that does not require transporting storage tanks with affixed applicators by means of robot arms to and from paint distributing docking systems, and which can be directly connected to paint supply tanks, all while maintaining complete voltage isolation from ground of the paint being sprayed.
The present invention provides such a system.